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J.RADIOANAL.NUCL.CHEM.,LETTERS 187 (5) 375-383 (1994) PHOTOCHEMICAL REDUCTION OF URANYL ION WITH TRIETHYLAMINE M.S. Sidhu, K.B. Kohli, P.V.K. Bhatia, S.S. Sandhu Department of Chemistry, Guru Nanak Dev University, Amritsar - 143 005, India Received 24 January 1994 Accepted I February 1994 Uranyl ion is photochemically reduced to uranium(IV) ~n the presence of triethyl- amine and triethylamine is oxidized to secondary amine and acetaldehyde. On the basis of product analysis, temperature independent quantum yields for uranium(IV) formation and abnormal Stern-Volmer plots rule out the simple collisional photo- chemical annihilation of excited uranyl ion with triethylamine. Static annihila- tion has a significant contribution in addition to dynamic annihilation. INTRODUCTION A variety of organic compounds efficiently annihilate electronically excited uranyl ion either through colli- sional electron transfer or a hydrogen atom abstraction mechanism I-5. Linear Stern-Volmer plots for guenching the excited uranyl ion with phosphorus(III)/sulfur(II) substrates involves collisional electron transfer to ex- cited uranyl ion followed by oxo-oxygen atom transfer to 375 Elsevier Science S. A., Lausanne Akaddmiai Kiad6, Budapest

Photochemical reduction of uranyl ion with triethylamine

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Page 1: Photochemical reduction of uranyl ion with triethylamine

J.RADIOANAL.NUCL.CHEM.,LETTERS 187 (5) 375-383 (1994)

PHOTOCHEMICAL REDUCTION OF URANYL ION WITH TRIETHYLAMINE

M.S. Sidhu, K.B. Kohli, P.V.K. Bhatia, S.S. Sandhu

Department of Chemistry, Guru Nanak Dev University, Amritsar - 143 005, India

Received 24 January 1994 Accepted I February 1994

Uranyl ion is photochemically reduced to uranium(IV) ~n the presence of triethyl- amine and triethylamine is oxidized to secondary amine and acetaldehyde. On the basis of product analysis, temperature independent quantum yields for uranium(IV) formation and abnormal Stern-Volmer plots rule out the simple collisional photo- chemical annihilation of excited uranyl ion with triethylamine. Static annihila- tion has a significant contribution in addition to dynamic annihilation.

INTRODUCTION

A variety of organic compounds efficiently annihilate

electronically excited uranyl ion either through colli-

sional electron transfer or a hydrogen atom abstraction

mechanism I-5. Linear Stern-Volmer plots for guenching

the excited uranyl ion with phosphorus(III)/sulfur(II)

substrates involves collisional electron transfer to ex-

cited uranyl ion followed by oxo-oxygen atom transfer to

375 Elsevier Science S. A., Lausanne Akaddmiai Kiad6, Budapest

Page 2: Photochemical reduction of uranyl ion with triethylamine

SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION

phosphorus(III)/sulfur(II) atoms, respectively, facili- 6-9

tated by d~-p~ interactions The present investiga-

tion involves the photochemical reduction of uranyl ion

with the nitrogen donor triethylamine. Positive devia-

tions from the straight line for Stern-Volmer plots

have been rationalized through dynamic and static 10

ground state complex formation

EXPERIMENTAL

Uranyl acetate, triethylamine, sulfuric acid and ace-

tone were A.R. Grade chemicals used as supplied. Deion-

ized doubly distilled water-acetone (30:70 v/v) was used

as reaction medium. Photolysis was carried out in a

pyrex glass photochemical reactor using 125 W medium

pressure mercury lamp (Hanovia Lamps). Quantum yields

were determined using a potassium ferrioxalate actinom-

eter (Table 1). The relative intensities of uranyl ion

luminescence at 506 nm were measured with a Perkin-

Elmer-LS-2B filter fluorimeter with excitation at ~ 345

nm. Electronic absorption spectra of photolyzed solu-

tions were recorded with a 240-UV Shimadzu recording

spectrophotometer, p-Nitroaniline and m-nitroaniline

fail to show a photochemical reaction but quench the

uranyl ion luminescence.

RESULTS AND DISCUSSION

Uranyl ion in acidic medium absorbs light in the

visible region (lmax = 420 nm). Addition of increasing

amounts of triethylamine (0.00-0.007M) does not shift

but gives a small depression in the absorbance at max

376

Page 3: Photochemical reduction of uranyl ion with triethylamine

SIDHU et ~.: PHOTOCHEMICAL REDUCTION OF URANYL ION

TABLE 1

Quantum yield and Stern-Volmer constants for uranium(IV) formation in photochemical reduction of

uranyl ion with triethylamine, [H +] = 0.2M at 30• ~

Sr. No. [N(C2H5)3], M [UO22+], M ~U(IV) Ksv, M-I

I 0.05 0.010 0.15

2 0.10 0.010 0.17

3 0.15 0.010 0.20

4 0.20 0.010 0.23

5 0.25 0.010 0.26

6 0.10 0.011 0.18

7 0.10 0.012 0.18

8 0.10 0.013 0.18

m - Nitroaniline

p - Nitroaniline

114.28

88.88

66.66

420 nm. However, further increase in triethylamine con-

centration (0.007"0.010M) leads to an enhancement in the

absorbance (Fig. I). Preferential protonation of tri-

ethylamine prevents its interaction with uranyl ion but

excess triethylamine interacts with uranyl ion because

of hard-hard interactions in the ground state and makes

the environment 0f the uranyl ion more rigid 11

Optical excitation of uranyl ion with visible light

renders the uranium-oxygen bond weaker and longer, since

an electron hops from the highest occupied molecular Or-

bital consisting of uranium 5f/6d and oxygen 2p atomic

orbitals to the lowest unoccupied molecular orbital con-

sisting of only uranium 5f orbital I'2'12'13 Triethyl-

amine is transparent to visible light in a pyrex glass

photochemical reactor. Consequently, absorbance of light

377

Page 4: Photochemical reduction of uranyl ion with triethylamine

SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION

u~ 0.415

8

0207

C 400 500 600 700

Wavelength, nm

Fig. I. Effect of increasing amounts of triethylamine (0.00, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009 and O.010M respec- tively) on the electronic absorption spectrum of uranyl ion (O.01M) in sulfuric acid (0.1M)

in the visible region makes the uranyl ion a labile

species, undergoing photochemical reduction to urani-

um(IV) (I = 650 nm) in the presence of triethylamine max

-1 (Fig. 2). A strong infrared band at 1705 cm with a

shoulder at 1750 cm -I due to the carbonyl stretching

frequency of acetic acid and ethyl acetate extracted

with ether after a 3 h irradiation of the solution con-

taining uranyl ion (0.04M) at 20• ~ Photochemical

oxidation of triethylamine with uranyl ion is similar

378

Page 5: Photochemical reduction of uranyl ion with triethylamine

SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION

0.130 u t - O

0

<

0.06- c

j "1,~,'--'=V" - ' . ~ ~" 400 500 600 700

Wavetength , nm

Fig. 2. Electronic absorption spectra of photolyzed solutions containing UO~ + (0.01M), H + (0.1M) ions and triethylamine ~0.01M) at time inter- vals of two minutes

to that observe with chlorine dioxide 14'15 The cation +

formed (CH3-CH=N(C2H5) 2) in aqueous solution yields sec-

ondary amine and acetaldehyde, which is photochemically 2,3

oxidized to acetic acid in the presence of uranyl ion

+ +

CH3-CH = N(C2H5) 2 + H20 ----> CH3CHO + H2N(C2H5) 2 (I)

The luminescence intensity of uranyl ion was monitored

in acetone-water medium with excitation at i = 345 nm and

379

Page 6: Photochemical reduction of uranyl ion with triethylamine

SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION

22

20 // ~r

Tmr ie tN~tYrloQan~ ~[ en e - ~ ' ~ / / 18- p-Nitroanilin],~ / / z x /

o " >/ i

Concentration ,xl0 -3

F i g . 3 . S t e r n - V o l m e r p l o t s f o r q u e n c h i n g u r a n y l i o n l u m i n e s c e n c e e m i s s i o n a t 5 0 6 nm w i t h X e x c i t a - t i o n = 345 nm

emission at I = 506 nm. The Stern-Volmer equation (Eq. 2)

If/If = I + kq T[Q] = I + Ksv[Q ] (2)

is applicable only at low concentration of triethylamine.

However, at higher concentration of triethylamine, a pos-

itive deviation from linearity (Fig. 3) rules out the

simple collisional deactivation of excited uranyl ion,

ground state charge transfer complex formation competing

with dynamic deactivation of uranyl iron by nitrogen

donor species (Scheme I). Aromatic z-electrons, play a 16 significant role in quenching uranyl ion luminescence

1 1 UO 2 + :N(C2Hs) 3 ~ ~.uv 2 -~-UO +~ H20+

+CH3-- CH = N(C2 H5 )2 (1 -5)hv a hvf Shy

K o ! +

UO 2*+ :N(C2Hs) 3 ~ (UO~ :N(C2Hs) 3)

380

Page 7: Photochemical reduction of uranyl ion with triethylamine

SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION

The relatively low value of the Stern-Volmer constant

for p-nitroaniline (Table I) may be due to its resonance

stabilization, where a positive charge is delocalized

on nitrogen atom in p-position but not in m-position to

the nitro group.

Here kf, kq and k_1 are the radiative decay, biomole-

cular diffusion and back diffusion rate constants, re-

spectively. K is the equilibrium constant for ground o

state complexation. Triethylamine is partitioned between

ground state and excited state of uranyl iron, forming a

ground state complex and exciplex, respectively. By vir-

tue of their position, the excited state complex is prone

to undergo immediate electron transfer from amine to

urany! ion.

Uranium(V) disproportionates to uranium(IV) and ura-

nium(VI) as soon as it in formed photochemically in

aqueous acidic medium 12'17

In summary, at low concentration of triethylamine,

preferencial protonation of the amine prevents it from

static complexation. However, at its higher concentra-

tion, static quenching plays a significant role in ad-

dition to dynamic annihilation of uranyl ions because

of hard-hard interaction and, consequently, uranyl ion

is reduced to uranium(IV) through an electron transfer

mechanism, followed by disproportionation of uranium(v)

in aqueous medium.

Financial assistance by CSIR, New Delhi (Scheme No.

5/157/89-EMR II) a~d research facilities to PV KB) by

Guru Nanak Dev University are gratefully acknowledged.

381

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SIDHU et al.: PHOTOCHEMICAL REDUCTION OF URANYL ION

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